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It seems having been firmly established that surface plasma waves (SPWs) could exist on any metal surfaces, including those of the ideal hard-wall type frequently employed in \textit{ab initio} studies of the dielectric responses of metals. Here we show that the surface waves hosted on hard-wall surface are not SPWs. Though they possess the celebrated frequency $\omega_p/\sqrt{2}$, where $\omega_p$ is the volume plasma wave frequency, these waves are shown devoid of charges in the infinite wavelength limit. In contrast, SPWs are always associated with a finite density of charges localized on the surface. This finding corrects a historical mistake and calls for a reappraisal of innumerable work that interpret experimental data (e.g. electron energy loss spectroscopy) based on the hard-wall model and related models such as the specular reflection model., Comment: This article is withdrawn because the conclusion needs to be modified (in progress), though the calculations remain correct

In a recent preprint, arXiv 2005.03716v1, G. Wegner and C. Henkel criticized my work on the possibility of lossless surface plasma waves. Here I refute all of their criticisms., Comment: Some sentences are rephrased to incorporate latest findings

We discuss surface roughness effects on the conduction of electrons in metals using both the quantal Kubo-Greenwood formalism and the semi-classical Fuchs-Sondheimer method. The main purpose here is to compare these methods and clarify a few subtle conceptual issues. One of such issues is concerned with the conditions under which the broken translation symmetry along a rough surface may be restored. This symmetry has often been presumed in existing work but not always with proper justifications. Another one relates to the physical meaning of a phenomenological parameter (denoted by $p$) intuitively introduced in the semi-classical theory. This parameter, called the specularity parameter or sometimes the \textit{Fuchs} parameter, plays an important role in the experimental studies of surface roughness but has so far lacked a rigorous microscopic definition. The third issue arises as to the domain of validity for the electrical conductivity obtained in those methods. A misplacement of the domain may have resulted in erroneous analysis of surface effects in a variety of electrodynamic phenomena including surface plasma waves., Comment: 10 pages, 1 figure

The Kovacs effect is a remarkable feature of the ageing dynamics of glass forming liquids near the glass transition temperature. It consists in a non-monotonous evolution of the volume/enthalpy after a succession of two abrupt temperature changes: first from a high initial temperature $T_i$ to a much lower annealing temperature $T_a$ followed by a smaller second jump back to a slightly higher final temperature $T_f$. The second change is performed when the instantaneous value of the volume/enthalpy coincides with the equilibrium one at the final temperature. While this protocol might be expected to yield equilibrium dynamics right after the second temperature change, one observes the so-called Kovacs hump in glassy systems. In this paper we apply such thermal protocol to the Distinguishable Particles Lattice Model (DPLM) for a wide range of fragility of the system. We study the Kovacs hump based on energy relaxation and all main experimental features are captured. Results are compared to general predictions based on a master equation approach in the linear response limit. We trace the origin of the Kovacs hump to the non-equilibrium nature of the probability distribution of particle interaction energies after the annealing and find that its difference with respect to the final equilibrium distribution is non-vanishing with two isolated zeros. This allows Kovacs' condition of equilibrium total energy to be met out-of-equilibrium, thus representing the memory content of the system. Furthermore, the hump is taller and occurs at a larger overlap with the system initial configuration for more fragile systems. The dynamics of a structural temperature for the mobile regions strongly depends on the glass fragility while for the immobile ones only a weak dependence is found., Comment: 9 pages, 9 figures: Added fragility study and encoding of the memory in the particles interaction energy distribution; added doi

Volume and enthalpy relaxation of glasses after a sudden temperature change has been extensively studied since Kovacs seminal work. One observes an asymmetric approach to equilibrium upon cooling versus heating and, more counter-intuitively, the expansion gap paradox, i.e. a dependence on the initial temperature of the effective relaxation time even close to equilibrium when heating. Here we show that a distinguishable-particles lattice model can capture both the asymmetry and the expansion gap. We quantitatively characterize the energetic states of the particles configurations using a physical realization of the fictive temperature called the structural temperature, which, in the heating case, displays a strong spatial heterogeneity. The system relaxes by nucleation and expansion of warmer mobile domains having attained the final temperature, against cooler immobile domains maintained at the initial temperature. A small population of these cooler regions persists close to equilibrium, thus explaining the paradox., Comment: Main text: 6 pages, 3 figures. Supplemental: 2 pages, 2 figures and 2 videos

We perform kinetic Monte Carlo simulations of a distinguishable-particle lattice model of structural glasses with random particle interactions. By varying the interaction distribution and the average particle hopping energy barrier, we obtain an extraordinarily wide range of kinetic fragility. A stretching exponent, characterizing structural relaxation, is found to decrease with the kinetic fragility in agreement with experiments. The most fragile glasses are those exhibiting low hopping barriers and, more importantly, dramatic drops of entropies upon cooling toward the glass transition temperatures. The entropy drops reduce possible kinetic pathways and lead to dramatic slowdowns in the dynamics. In addition, the kinetic fragility is shown to correlate with a thermodynamic fragility., Comment: Main text: 6 pages with 4 figures Supplementary: 11 pages with 18 figures

We study the electrostatic responses (i.e. retardation effects due to the propagation of electromagnetic waves are ignored) of a linear homogeneous and anisotropic (LHA) dielectric film to an arbitrary external electrostatic potential, which may be generated by charges located either inside or outside it. A formalism is developed to calculate the polarization charges induced in the film. In our derivation, the idea is exploited that a physical boundary can be looked upon as a region of rapid variation in polarization rather than a simple geometric separation. With this no boundary conditions are needed in solving the relevant electrostatics problem. Our approach makes it clear that the responses consist of two contributions, one arising from the very presence of surfaces while the other existing even in an infinite medium. The approach can be applied not just to electrostatic but also many problems and is of great pedagogical value. In light of the results, we discuss graphene plasma waves under the influence of a LHA dielectric film such as a few-layer hexagonal boron nitride. It is found that the dispersion of these waves is strongly affected by the anisotropy at wavelengths comparable to the film thickness., Comment: 20 pages, 4 figs

We report an analytical evaluation of the mean-squared displacement (MSD) of the particles in glasses based on their coarse grained trajectories. The calculation is conducted by means of a local random configuration-tree theory that was recently proposed by one of us [C.-H. Lam, J. Stat. Mech. \textbf{2018}, 023301 (2018)]. Results are compared with the numerical simulations of a lattice glass model, and good quantitative agreement has been obtained over a wide range of temperatures in the entire region of time with virtually no free parameters. To the best of our knowledge, the calculation is the first in its kind., Comment: 11 pages, 2 figures

Topological electric quadrupole is a recently proposed concept that extends the theory of electric polarization of crystals to higher orders. Such a quadrupole phase supports topological states localized on both edges and corners. In this work, we show that in a quadrupole phase of honeycomb lattice, topological helical edge states and pseudo-spin-polarized corner states appear by making use of a pseudo-spin degree of freedom related to point group symmetry. Furthermore, we argue that a general condition for emergence of helical edge states in a (pseudo-)spinful quadrupole phase is mirror or time-reversal symmetry. Our results offers a way of generating topological helical edge states without spin-orbital couplings.

We report a general macroscopic theory for the electrodynamic response of semi-infinite metals (SIMs). The theory includes the hitherto overlooked capacitive effects due to the finite spatial extension of a surface. The basic structure of this theory is independent of the particulars of electron dynamics. Analytical expressions have been obtained of the charge density-density response function, which is naturally parsed into two parts. One of them represents a bulk property while the other a pure surface property. We apply the theory to study the responses according to several electronic dynamics models and provide a unified view of their validity and limitations. The models studied include the local dielectric model (DM), the dispersive hydrodynamic model (HDM) and specular reflection model (SRM), as well as the less common semi-classical model (SCM) based on Boltzmann's transport equation. We show that, in terms of their basic equations, the SRM is an extension of the HDM, just as the HDM is an extension of the DM. The SCM improves over the SRM critically through the inclusion of translation symmetry breaking and surface roughness effects. We then employ the response function to evaluate the so-called dynamical structure factor, which plays an important role in particle scattering. As expected, this factor reveals a peak due to the excitation of surface plasma waves (SPWs). Surprisingly, however, the peak is shown to be considerably sharper in the SCM than in other models, indicating an incipient instability of the system according to this model. We also study the distribution of charges induced by a charged particle grazing over a SIM surface at constant speed. This distribution is shown to contain model-specific features that are of immediate experimental interest., Comment: 24 pages, 10 figures, a few more references are added and discussed, abstract and introduction modified to improve presentation over older versions, more experimental aspects are discussed

Recently, we have revealed an intrinsic instability of metals due to surface plasma waves (SPWs) and raised the prospect of using it to create lossless SPWs. The counter-intuitive nature of this finding prompts one to ask, why had not this instability been disclosed before, given the long history of this subject? If this instability does exist, how far is it from reality? The present work is devoted to answering these questions. To this end, we derive a unified macroscopic theory of SPWs that applies to any type of electron dynamics, be they local or non-local, classical or quantum-mechanical. In light of this theory, we analyze the behaviors of SPWs according to several electron dynamics models, including the widely used local dielectric model (DM), the hydrodynamic model (HDM) and the specular reflection model (SRM), in addition to the less common semi-classical model (SCM). We find that, in order to unveil the instability, one must (i) self-consistently treat surface effects without any of the usually imposed auxiliary conditions and (ii) include translation symmetry breaking effects in electron dynamics. As far as we are concerned, none existing work had fulfilled both (i) and (ii). To assess the possibility of realizing the instability, we analyze two very important factors: the dielectric interfacing the metal and inter-band transitions, which both were ignored in our recent work. Whereas inter-band absorption -- together with Landau damping -- is shown adverse to the instability, a dielectric brings it closer to occurrence. One may even attain it in common plasmonic materials such as silver under not so tough conditions., Comment: 16 pages, 6 figures, title changed, rewrite of the Introduction and abstract, restructuring, Fig. 2 deleted, Appendix B added, more references added, material in Sec. VI has some overlap with arXiv:1706.03404 (not to be submitted)

Superconductivity is a result of quantum coherence at macroscopic scales. Two superconductors separated by a metallic or insulating weak link exhibit the AC Josephson effect - the conversion of a DC voltage bias into an AC supercurrent. This current may be used to activate mechanical oscillations in a suspended weak link. As the DC voltage bias condition is remarkably difficult to achieve in experiments, here we analyse theoretically how the Josephson effect can be exploited to activate and detect mechanical oscillations in the experimentally relevant condition with purely DC current bias. We unveil for the first time how changing the strength of the electromechanical coupling results in two qualitatively different regimes showing dramatic effects of the oscillations on the DC current-voltage characteristic of the device. These include the apperance of Shapiro-like plateaux for weak coupling and a sudden mechanically-induced retrapping for strong coupling. Our predictions, measurable in state of the art experimental setups, allow the determination of the frequency and quality factor of the resonator using DC only techniques., Comment: 10 pages, 6 figures

An elementary approach is employed to show that genuine surface effects could destabilize surface plasma waves (SPWs) supported on the interface between a metal and a dielectric (usually the vacuum) and give rise to an intrinsic channel of gain for these waves. A comprehensive SPW theory is presented taking into account both the inter-band transition effects and the dielectric effects. Experimental consequences, especially in regard to the possibility of overcompensating for the energy losses suffered by SPWs, are exemplified for two common plasmonic materials: silver (Ag) and aluminum (Al). In both metals, the intrinsic channel of gain is shown having substantially reduced but insufficient to overcompensate for the losses, for different reasons. In Ag, the inter-band transition effects significantly weaken the intrinsic gain channel and make it unable to overcompensate for the losses, while in Al it is because the loss rate is too big. Nevertheless, we find it possible to enhance the intrinsic gain rate by replacing the vacuum with a moderate dielectric so that the losses can be overcompensated in Ag. This prediction is ready for experimental exploitation., Comment: 10 pages, 5 figs

We present a theory of surface plasma waves in metal films with arbitrary electronic collision rate $\tau$. Both \textit{tangential} and \textit{normal} modes are investigated. A universal self-amplification channel for these waves is established as a result of the unique interplay between ballistic electronic motions and boundary effects. The channel is shown to be protected by a general principle and its properties independent of $\tau$. The effects of film thickness and surface roughness are also calculated. Experimental implications, such as Ferrel radiation, are discussed., Comment: 14 pages, 3 figs. arXiv admin note: text overlap with arXiv:1701.01060

We present a theory of surface plasma waves (SPWs) in metals with arbitrary electronic collision rate $1/\tau$. We show that there exists a universal intrinsic amplification channel for these waves, as a result of the unique interplay between ballistic electronic motions and the metal surface. The corresponding intrinsic amplification rate $\gamma_0$ is shown to be independent of $\tau$. We also study its dependence on surface scattering properties., Comment: 10 pages, 4 figs